1. Field of the Invention
The present invention relates to a technique for driving a print head of an ink-jet printer.
2. Description of the Related Art
In a print head driving method in a conventional drop on-demand type ink-jet printer, a drive voltage signal is applied to a print head every time a print instruction is issued, and ink drops are discharged from a nozzle to carry out a print operation.
FIG. 1 is a diagram showing an example of the structure of the print head of an ink-jet printer. As shown in FIG. 1, the print head 12 is composed of an ink discharging nozzle 14, an ink pressure increasing room 16, an actuator 18, and a drive signal generating circuit 19. The ink pressure increasing room 16 is connected to the nozzle 14. The actuator 18 receives a pulse drive voltage signal and applies a pressure to ink in the ink pressure increasing room 16 in accordance with the magnitude of a drive voltage signal. The drive signal generating circuit 19 generates the drive voltage signal which should be applied to the actuator 18.
The print head 12 is subjected to a repetitive reciprocating motion in a print region along a paper (not shown). In this state, the pulse drive voltage signal is generated by the drive signal generating circuit 19 and is repeatedly supplied to the actuator 18. As a result, the ink in the ink pressure increasing room 16 is pressurized so that ink drops are discharged from the nozzle 14 to the paper. The supply and non-supply of the pulse drive voltage signal generated by the drive signal generating circuit 19 to the actuator 18 are controlled so that a print operation to the paper is carried out.
FIGS. 2A to 2C are waveform diagrams showing the waveform of the drive voltage signal, the displacement of ink meniscus at the nozzle tip section and the velocity of an ink meniscus at the nozzle tip section, respectively. In FIGS. 2A to 2C, the horizontal axis indicates time and a vertical axis indicates voltage in FIG. 2A, the displacement of the meniscus in FIG. 2B, and the meniscus velocity in FIG. 2C, respectively.
When the drive voltage signal is supplied to the actuator 18 as shown in FIG. 2A, the drive voltage signal increased rapidly between a point A and a point B, and ink 20 in the ink pressure increasing room 16 is also pressurized rapidly by the actuator 18. At this time, the meniscus velocity in the nozzle tip section is rapidly increased between a point X and a point Y in FIG. 2C. The ink meniscus 22, 21 changes from the original state shown in FIG. 3A to the state shown in FIG. 3B, and the discharge of ink drop from the tip section of the nozzle 14 is started. Thus, an ink pillar 24 is first formed. At this time, the displacement quantity of the meniscus 22 becomes large rapidly as shown in FIG. 3B.
After that, the drive voltage signal is settled to a constant value between the point P and a point C in FIG. 2A. As a result, the pressure of the meniscus 22 decreases and the velocity of the ink meniscus in the nozzle tip section starts to decrease between a point Y and a point Z in FIG. 2C. Thus, the difference in meniscus velocity between the ink pillar 24 discharged from the nozzle 14 and the ink within the nozzle becomes large. For this reason, as shown in FIG. 3C, the ink pillar 24 is cut off from the ink within the nozzle 14 and an ink drop 26 is discharged from the nozzle 14.
It should be noted that the drive voltage signal is sometimes decreased depending on a printer, instead of keeping constant between the point B and the point C shown in FIG. 2A. In the case, the drive voltage signal is decreased at the timing earlier than the velocity of the ink meniscus. However, the basic operation is the same.
After the ink drop 26 is discharged, the position of the meniscus 22 in the tip section of nozzle 14 is recessed to the side of the nozzle proximate by a quantity equivalent to discharged ink drop, as shown in FIG. 3D.
After that, the recessed meniscus 22 tries to return to the original position by surface tension in the tip section of the nozzle 14 and vibrates (refill phenomenon). Also, the recessed meniscus 22 undergoes influence of the remaining vibration of the pressure wave by the actuator 18. Thus, the meniscus vibrates. The vibration attenuates gradually and the meniscus 22 returns to the original position as shown in FIG. 2B, and FIG. 3A. Also, the velocity of ink meniscus attenuates gradually and becomes a zero, as shown in FIG. 2C. Such an operation is repeated every time the drive voltage signal is supplied to the actuator 18 and the print operation is carried out.
By the way, the ink meniscus in the nozzle section vibrates for a time as mentioned above, when the drive voltage signal is once supplied and the ink drop is discharged. Therefore, the ink drop can be next discharged at the timing of Q in FIG. 2A or after that. That is, the next discharge of the ink drops is after the vibration of the ink meniscus 22 has been settled. However, it is impossible to print at high speed, because the supply period of the drive voltage signals becomes long in the above-mentioned condition. For this reason, it could be considered that the drive voltage signal is supplied to the actuator 18 before the timing of Q and ink drops are discharged.
For example, as shown in FIG. 2B, the position of the ink meniscus returns to the original position at the timing of A in FIG. 2A. Therefore, it is effective to supply the drive voltage signal at this timing. However, at the timing of O, the ink meniscus is moving with some velocity as shown in FIG. 2C. Therefore, when the drive voltage signal is supplied at this timing, the velocity of the ink meniscus is equal to an addition of the above remaining velocity and a velocity determined in response to the new drive voltage signal. This is different from the desired velocity and causes the degradation of the print quality.
On the other hand, as shown in FIG. 2C, the ink meniscus stops at the timing of P. In this case, if the following drive voltage signal is supplied, there is no problem with respect to the meniscus velocity of the ink. However, as shown in FIG. 2B, because the ink meniscus is displaced largely, the quantity of discharged ink drop is different from the desired quantity and still causes the degradation of the print quality.
To solve these problems, various methods are conventionally proposed, in which after the drive voltage signal is once supplied, a preliminary drive voltage signal is supplied to control the ink meniscus and makes high-velocity print possible.
Also, various methods are proposed in which a quantity of ink discharged from the nozzle 14 is increased through the once drive of the actuator 18 so that the discharge efficiency of ink is improved.
Hereinafter, three representative methods will be described.
In the first conventional method, the ink meniscus 22 at the tip section of the nozzle 14 is returned to an initial state, i.e., the state before the ink drops are discharged, as soon as possible. The repetition period of the supply of the drive voltage signal is made small. For this purpose, the drive voltage signal is supplied to restrain the remaining vibration of the meniscus 22 after supply of the drive voltage signal. For example, the first conventional method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-338150) and Japanese Laid Open Patent Application (JP-A-Showa 59-10495).
FIG. 4A is a waveform diagram showing the waveform of the drive voltage signal and the preliminary drive voltage signal in the first conventional driving method. FIG. 4B is a waveform diagram showing the drive voltage signal and the preliminary drive voltage signal which are repeatedly supplied to the actuator 18 in the first conventional driving method.
As shown in FIG. 4A and 4B, in the first conventional driving method, every time the drive voltage signal 28 is supplied to the actuator 18 to discharge the ink drops, the preliminary drive voltage signal 30 is supplied to restrain the remaining vibration of the ink meniscus immediately after. Therefore, the drive voltage signal 28 is once supplied and then the ink meniscus 22 is always reset to the initial state at the timing when the next drive voltage signal is supplied, i.e., at the timing shown by a dotted line in FIG. 4B.
It should be noted that when the preliminary drive voltage signal is supplied to the actuator 18, the velocity of ink meniscus in the nozzle 14 changes as shown in FIG. 2C, like the case where the drive voltage signal is supplied. However, in case of the supply of the preliminary drive voltage signal, the change of the meniscus velocity between the points P and Q in FIG. 2C is small, even if the ink pillar 24 protrudes from the nozzle 14. Therefore, the ink pillar 24 is not cut off and the ink drop is not discharged, as shown in FIG. 3C.
In the second conventional driving method, the ink drops are discharged using the vibration of the ink meniscus so that the discharge efficiency of ink is improved. For this purpose, the preliminary drive voltage signal is supplied before the drive voltage signal is supplied (Japanese Laid Open Patent Applications (JP-A-Heisei 5-338148, JP-A-Heisei 5-318766, and JP-A-Heisei 9-29959).
FIG. 5A is a waveform diagram showing waveforms of the preliminary drive voltage signal and drive voltage signal in the second conventional driving method. FIG. 5B is a waveform diagram showing the preliminary drive voltage signal and the drive voltage signal which are repeatedly supplied to the actuator 18 in the second conventional driving method.
As shown in FIGS. 5A and 5B, in the second conventional driving method, the preliminary drive voltage signal 29 is always supplied before the drive voltage signal 31 is supplied to the actuator 18. Therefore, the supply of preliminary drive voltage signal 29 vibrates the ink meniscus 22, and the drive voltage signal 31 is supplied at the timing of substantially the same vibration state and the ink drops are discharged from the nozzle 14.
In a third conventional driving method, the solvent of ink volatilizes in the tip section of the nozzle 14, so that it is easy to increase the viscosity coefficient of ink. When the viscosity coefficient of ink is increased, the characteristic of the discharge of ink drops changes largely and causes the degradation of the print quality. In the third conventional driving method, increase in viscosity coefficient of ink is prevented. For this purpose, the print head 12 is regularly moved to the nozzle cleaning mechanism section and the ink drops are discharged, (Hereinafter, this operation will be referred to as a purge). Also, the drive voltage signal is supplied to the extent to which the ink discharge is not carried out, when the print head 12 is in the non-print region (Japanese Patent Application No. Heisei 2-195868).
Moreover, a method is proposed in Japanese Laid Open Patent Application (JP-A-Heisei 9-226116). In this reference, the preliminary drive voltage signal is supplied before the drive voltage signal is supplied to the actuator 18, when the ink drops are discharged during the period next to the period during which any ink drops are not discharged.
As described above, the first conventional driving method is the method of reducing the drive period for the actuator 18 or increasing an drive frequency. Also, the second conventional driving method is the method of increasing the discharge efficiency of ink. By these methods, there is a case where both of the increase of the drive frequency for the actuator 18 and the increase of the discharge efficiency of ink should be attained at the same time. In such a case, the preliminary drive voltage signal. is first supplied to preliminarily vibrates the ink meniscus, and then the drive voltage signal is supplied to discharge the ink drops. Next, the preliminary drive voltage signal is supplied to restrain the remaining vibration of the ink meniscus 22.
It is supposed that the first and second drive voltage signals are supplied to the actuator 18 during the current period and the next period. In this case, the preliminary drive voltage signal is supplied to the actuator 18 between the first drive voltage signal and the second drive voltage signal to restrain the vibration of the ink meniscus 22. Then, the preliminary drive voltage signal is supplied to the actuator 18 between the first drive voltage signal and the second drive voltage signal to vibrate the ink meniscus 22 preliminarily. That is, it is necessary to carry out a wasteful operation in which the vibration of the ink meniscus 22 is again started after the remaining vibration of the ink meniscus is once restrained. Thus, the drive frequency of the actuator 18 is rather decreased.
Also, it could be considered that the preliminary drive voltage signal is only once supplied in case of the continuation drive. In this case, however, it is necessary to manage whether or not the ink drops have been discharged during the previous period, i.e., to manage whether or not the continuation drive is carried out. Thus, a new problem is caused that the supply control of the drive voltage signal becomes complicated.
In the third conventional driving method, the preliminary drive voltage signal is supplied to the actuator 18 under the condition that the print head 12 is moved to the non-print region to make it possible to prevent the increase of the viscosity coefficient of ink in the tip section of the nozzle 14 as mentioned above. However, a case where the printing operation is carried out only in a top or bottom section in the print region could be considered as an example. In this case, the ink drops are not discharged while the print head 12 is moved to the bottom section of the print region, after the ink drops are discharged at the head section of the print region. Therefore, the evaporation of ink solvent increases the viscosity coefficient of ink until the print head reaches the bottom section of the print region, so that it is easy for the print quality to be degraded.
Also, when the ink drops are discharged at the next period to the current period during which any ink drops are not discharged, it is necessary to manage whether or not the ink drops are discharged at the previous period, in the method of supplying the preliminary drive voltage signal to the actuator 18. In this case, a new problem is caused that the supply control of the drive voltage signal becomes complicated, like the above-mentioned case.
In conjunction with the above description, an ink-jet type printer is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-218928). In this reference, a drive signal circuit (49) generates a first voltage to make a piezo-electric vibrator extend at a velocity suitable for formation of ink drops, a second voltage to hold an expanded state or a shrunk state of the piezo-electric vibrator, and a third voltage to make the piezo-electric vibrator shrink at a velocity suitable for attraction of ink into a pressure generating room. A discharge end detecting circuit (52) detects a timing when an ink drop generating process is ended in response to the first voltage. A delay circuit (53) delays a signal outputted from the circuit (52) by a time xcex94T until the vibration of meniscus produced in the ink drop generating process is switched into a motion to a nozzle opening. A charge signal generating circuit (48) drives the circuit (49) to generate the third voltage in response to the delayed signal from the delay circuit (53). A discharge signal generating circuit (51) drives the circuit (49) to generate the first voltage in response to a print timing signal.
Therefore, an object of the present invention is to provide an apparatus of driving a print head in an ink-jet printer, in which a drive period of an actuator can be made short with a simple control, resulting in improvement of a print speed.
Another object of the present invention is to provide an apparatus of driving a print head in an ink-jet printer, in which a condition of the discharge of ink can be held constant, resulting in improvement of print quality.
Still another object of the present invention is to provide an apparatus of driving a print head in an ink-jet printer, in which the discharge efficiency of ink is improved so that print quality can be improved.
Yet still another object of the present invention is to provide an apparatus of driving a print head in an ink-jet printer, in which it is possible to prevent increase in viscosity coefficient of ink so that print quality can be improved.
In order to achieve an aspect of the present invention, an ink-jet printer includes a nozzle, an ink storage room, an actuator and a drive section. Ink drops are discharged from the nozzle in a print operation. The ink storage room stores ink. The actuator applies pressure to the ink stored in the ink storage room for the ink drops to be discharged in response to each of a drive signal and a preliminary drive signal. The drive section selectively issues one of the drive signal and the preliminary drive signal to the actuator for each of unit time periods, based on whether or not the ink drops should be discharged. The unit time period is shorter than a time period needed until vibration of an ink meniscus in an end portion of the nozzle is attenuated.
Here, the drive section issues the drive signal to the actuator at a start timing of a print unit time period of the unit time periods when the ink drops should be discharged in the print unit time period.
Also, the drive section issues the preliminary drive signal to the actuator at a predetermined timing of a non-print unit time period of the unit time periods when the ink drops should not be discharged in the non-print unit time period. In this case, it is preferable that a time from a start timing to the predetermining timing in one of the unit time periods is longer than a time from a start timing to a timing when the drive signal is issued, in another of the unit time periods. Also, the predetermining timing in a current one of the unit time periods may be determined based on a vibration waveform of the ink meniscus when the drive signal is issued in one of the unit time periods immediately previous the current unit time period.
Also, the drive section determines a waveform of the preliminary drive signal and a timing of issuance of the preliminary drive signal such that vibration of the ink meniscus at a start timing of a next unit time period to a current unit time period when the drive signal is issued in the current unit time period is substantially the same as that of the ink meniscus at the start timing of the next unit time period when the preliminary drive signal is issued in the current unit time period in place of the drive signal. In this case, the drive section includes a drive circuit determining the waveform of the preliminary drive signal, and a timing setting circuit setting a timing at which the preliminary drive signal should be issued.
Also, a print head includes the nozzle, the ink storage room and the actuator, and the drive section issues the preliminary drive signal in one of the unit time periods immediately before the print head is moved from an outside of a print region into an inside of the print region.
In order to achieve another aspect of the present invention, a method of driving a print head of an ink-jet printer, includes:
issuing a drive signal in a current time period such that ink drops are discharged for a print operation, an ink meniscus having remaining vibration after the discharging of the ink drops; and
issuing the drive signal in a next time period to the current time period, wherein the remaining vibration of the ink meniscus is not fully attenuated.
Here, the drive signal is issued at a start timing of the current unit time period.
Also, the method may further include issuing a preliminary drive signal at a predetermined timing of the next time period when the ink drops are not discharged. In this case, it is preferable that a time from a start timing to the predetermining timing in the next time period is longer than a time from a start timing to a timing when the drive signal is issued, in the next time period. Also, the predetermining timing is determined based on the remaining vibration of the ink meniscus when the drive signal is issued in the current time period.
Also, a waveform of the preliminary drive signal and a timing of the issuance of the preliminary drive signal are determined such that the remaining vibration of the ink meniscus in a tip section of a nozzle at a start timing of the next unit time period is substantially the same as that of the ink meniscus at the start timing of the next time period when the preliminary drive signal is issued in the current time period in place of the drive signal.
Also, the method may further include issuing the preliminary drive signal in a time period immediately before a print head is moved from an outside of a print region into an inside of the print region.